The present invention relates generally to a method and apparatus for installing and servicing an array of individual sensor pods used, for example, for seismic measurement of a hydrocarbon field.
The value of seismic surveying to the exploration and discovery of oil and gas formations is well known in the petroleum industry. Developments in computing power have enabled the further development and use of seismic surveying including three dimensional (3-D) seismic surveys. In many cases, the processing of the measured seismic data can be a critical factor in the value of a seismic survey. The final interpretation of a seismic image is only as good as the quality of the seismic data.
Today, 3-D subsea seismic exploration is typically accomplished with a seismic survey ship equipped with an energy source and a plurality of receivers attached to one or more streamers that are towed behind the ship at a very precise speed. The energy source is designed to produce compressional waves that propagate through the water and into the underwater land formation. As the compressional waves propagate through the land formation, they strike interfaces between the formations, commonly referred to as strata, and reflect back through the earth and water to the receiver. The receivers typically function to convert the reflected waves into electrical signals that are then processed into an image that provides information about the structure of the subterranean formation.
It has been discovered that a subsea array of individual sensor pods resting on the sea floor more accurately measures the 3-D seismic data obtained for a hydrocarbon field. To form such an array, individual sensor pods must typically be located in a grid on the sea floor at precise locations, typically about 200–500 meters apart. Sensor pod locations may be measured with a long base line acoustic array and transponders. Each sensor pod contains a data collection computer and an internal battery. After the array is in place, a surface ship is used to provide the acoustic source and the sensor pods record the subsequent reflections. At regular intervals the sensor pods must be retrieved and replaced with fresh pods. On the surface, the data stored in each recovered sensor pod must be downloaded and the battery recharged before it can be reused. Installing and replacing the sensor pod array is a significant part of the seismic survey because the survey cannot start or resume until all sensor pods are in place. In water depths of 3,000 meters, for example, travel time between the surface and the sea floor is significant, so an efficient method for placing and retrieving sensor pods is critical to an economically successful subsea seismic survey.
What is needed is a method and apparatus for transporting, installing, and retrieving a sensor array of individual sensor pods at a geographically remote location, such as on the sea floor.